I've found a glitch with the message grouping. I have this set: Messages -> Options -> Message Display & Actions -> Display Style -> Group by Subject.
When I receive a message sent to several people, and I delete the original message, and then someone replies ALL, I get a grouping that includes the first message. When I delete the group, the 2nd message is deleted, but the first message comes back and I cannot delete it (again).
The only way to make the original message disappear from my Messages list is to reboot.
No longer a BES Admin, but it was fun while it lasted!